oldstable - The previous stable release (Jessie).stable - The current stable release (Stretch).testing - The next generation release (Buster).unstable - The unstable development release (Sid), where new or updated packages are introduced.

Unstable is always called Sid. Once testing becomes stable, stable will be called Buster, oldstable Stretch and Jessie gets retired.

You can use both the release cycle and release name in your repositories configuration. So you can configure a repository on "testing" and that will into perpetuity refer to the current testing release. You can also use "buster", which while currently referring to the exact same packages as "testing" would, it would shift to point to the packages in "stable" repository once Buster is released as stable. There's a case for both but yeah, this can get confusing

Only for the sake of completeness is information, self-experience. I mean, the experimental mirror (Debian) can not be used with the apt-pinning method, but no priority can be given. Of course, the testing mirror is more than enough for newer packages, but as an idea, the experimental mirror may arise.

I've updated the topic title and the tutorial itself to make clear it should no longer be used.

This is an older tutorial and won't work correctly without changes today. That's because at the time of writing this tutorial the Jessie repository was Debian stable and the Stretch repository was Debian testing. LMDE 2 is based on Jessie. However, since mid last year Jessie was relegated to Debian old-stable, Stretch was promoted to Debian stable and Buster became the new repository for Debian testing. Where in the tutorial it says "testing" or "stable", that no longer is correct and will not give desired results.

Do not use this tutorial without adapting it to current situation. I'll provide a new tutorial once LMDE 3 is released and will write it such that it is more future proof than this tutorial has proven to be.